In injection molding, upper and lower mold halves are brought together to define a mold cavity into which heated molten plastic is injected under pressure. The mold halves are typically vertically aligned with an upper mold portion termed the cavity half and a lower mold portion termed the core mold half. For forming holes or apertures through the top or bottom surfaces of molded pieces, coring elements, such core pins are frequently utilized. When a raised aperture having side walls extending from the surface of the molded part is desired, the core element is often provided with an associated ejector sleeve which assists in forming a lip of the raised aperture and in breaking the plastic part free from the coring element to eject the part as the mold is opened.
Core pins are long, thin, metallic or ceramic cylinders which have a base with a head and shoulder portion adapted to assist in retaining the pin in the mold section or press piece. The pin's tip, which extends into the mold cavity, may be machined to a smaller diameter which forms a shoulder and is typically drafted so that the pin is tapered at the end to assist in ejection. The sleeve is typically a hollow tube sized to fit over the pin which has a base portion having head and shoulder portions for retention in a counterbore formed in the ejector plate. Core pins are typically mounted in the core mold half. When mounted, the pin extends through an aperture formed in the mold half and into the mold cavity.
In forming a raised aperture, the interior surface or side walls of the aperture are defined by the surface of the core pin, the lip of the raised aperture is defined by the core pin ejector sleeve (hereinafter “sleeve”) and optionally a machined shoulder on the core pin, and the exterior surface of the side walls of the aperture are defined by the core mold half.
The diameter and shape of the interior of the raised aperture is determined by the mold's end user by machining the diameter and shape of the end of the core pin and thickness of the sleeve as desired to correspond to the raised aperture's intended purpose. For example, if the raised aperture is placed in the molded article in order to receive a ⅛th screw, the diameter of the pin must be approximately ⅛th, approximately cylindrical in shape and within acceptable tolerances. The width of the side walls of the raised aperture is determined by the width of the sleeve and the cross-sectional diameter of the shoulder aperture in the mold. The width of the walls of the hole formed in the molded article is set by the end user by considering a number of factors, including the strength and rigidity of the plastic used to form the article.
Given the long, slender shape of the pin and sleeve, the core pin and sleeve can become broken or bent during installation or during the molding cycle. This damage may occur when the sleeve or molded article catch upon the sides of the aperture or are not properly positioned when the mold is closed. When the pin or ejector sleeve are damaged, the mold may become jammed and the mold machine must be taken off-line, disassembled and repaired. At the very least, this repair requires that the broken core pin be replaced, while loose fragments of the broken pin and/or molded articles must be removed. These repairs increase the maintenance costs for the injection mold machine and cause the mold's end user to lose valuable production time.
Another problem with prior core pins and sleeves is that when the molded article is ejected from the mold, the walls of the hole formed in the article can suffer fracture or defect due to adhesion of uncured plastic to the mold, core pin or sleeve. This damage can be due to the side walls having insufficient time to cure or cool during the mold cycle. One way to reduce molding cycle time is to reduce the thickness of the side wall of the raised aperture. In hot plastic and thermosetting plastic mold injection, any increase in the amount of plastic used to form a raised aperture also increases the cooling/setting cycle time for the mold, wherein the plastic cools and hardens to gain sufficient rigidity to prevent fracture or deformation prior to ejection. The more material used, the longer it takes to cool sufficiently. Even a small increase in cycle time per piece can become a significant manufacturing cost when considering the number of articles being produced. Thus, it is generally recognized as desirable to decrease the side wall thickness of a raised aperture. Decreasing the width of the walls in turn decreases the amount of plastic used to fill the mold cavity and form the raised aperture. Even if this amount is negligible when viewed on a per/piece or per hole amount, since the number of pieces produced by a given mold can range into millions, and since there are often multiple raised apertures formed per molded article any decrease in materials used can result in significant cost savings for the end user.
Standard sleeve thicknesses for stock sleeves have commonly ranged from about 0.046 to about 0.125 inch which can form a raised aperture with side-wall thickness ranges from 0.046 to 0.125 inches. The foregoing advantages have resulted in mold users seeking to use thinner sleeves to make raised apertures with the same internal diameter, but with thinner side walls. However, mold component manufacturers have been reluctant to provide thinner sleeves due to their perceived fragility, particularly when the end user wants a sleeve of considerable length, e.g. a sleeve that is more than four inches long. In order to decrease the thickness of the side-wall of a raised aperture while maintaining the same dimension for its inner diameter, a thinner sleeve is required.
The fragility of current injection mold core pin and sleeves can be exacerbated by the manner in which the pin is mounted. In one common type of mount, a threaded retaining bolt is used to secure the pin base to a press piece of the core mold half. The core pin base typically includes a broadened head and shoulders. A threaded mounting aperture is formed in the press piece to receive the base of the pin and retaining bolt. The retaining bolt is threaded into the mounting aperture so that the shoulder on the base of the pin is held flush against the top surface of the mounting aperture. This results in the mounting shoulder of the core pin being held tight against the interior surface of the press piece. Such a rigid hold on the base of the pin does not allow for any movement of the pin or sleeve relative to the molded part which can cause the pin and/or sleeve to be damaged during ejection of the part.
To alleviate the problems with rigid mounting, a second type of mount for core elements has been used which employs a separate mounting plate. The separate mounting plate is held in place by a separate mounting screw or bolt. The plate holds the pin in place, but with head clearance between the pin head and the press piece which allows the core pin and sleeve to move or float in its mounting. Such movement helps prevent damage to the pin and sleeve if the pin or sleeve should catch upon the molded part of a portion of the mold during travel. However, this type of plate mount suffers from several drawbacks. For example, a relatively large rectangular aperture is typically required to be machined or milled into the exterior surface of the press piece to accommodate the mounting plate. The aperture for retaining the core pin head also has to be machined into the mold, as does a separate mounting hole for the bolt or screw which holds the mounting plate in place. Finally, a hole must also be machined into the mounting plate itself to correspond to the hole in the mold where the screw or bolt joins the plate and mold. This process adds several additional machining steps to the design and manufacture of the injection mold thereby increasing the mold's production cost. Further, since useable space for fitting required components on a mold is usually limited, the floating mount plate is often viewed as an inefficient use of that available space on the mold.
Another problem with prior ejector sleeves was that they become worn quicker than is desirable by friction between the exterior surface of the sleeve and the mold base as well as between the interior surface of the sleeve and the exterior surface of the core pin. Thus, there is a need for a core pin and core pin sleeve which have improved wear characteristics.
Thus, there is a need for a more robust, thin walled sleeve which can be more easily installed and which can be used to form a thin walled raised apertures in injection molded articles thereby reducing cycle time and providing a savings on material costs.